The present invention relates to semiconductor optoelectronics, and more specifically to integration of photonic devices in the front-end of the line stack (FEOL) of a complementary metal-oxide semiconductor (CMOS) structure of a wafer chip.
Photonic structures can be fabricated on wafer chips in order to create wafers that operate both in an electronic domain and an optical domain. When an optical fiber is used to input light into a waveguide on a wafer chip, care must be taken to properly manage the polarization of light. The orientation of the polarization state in an optical fiber changes randomly with time. The performance of photonic devices on wafer chips is very sensitive to the orientation of the polarization state. Hence, the input polarization state must be processed on the wafer chip for it to be re-oriented into the polarization state for which the photonic devices work the best. To achieve such polarization re-orientation, a polarization splitter and rotator (PSR) is used.
Generally, wafer manufacturing includes a front-end of the line (FEOL) process during which devices are built into bottom layers of a CMOS stack of the wafer chip and a back end of line (BEOL) process for building top layers of the wafer chip that include wiring that interconnects the devices formed during the FEOL process. The bottom FEOL layers include transistors of the wafer chip and its various components (i.e., source, drain, gate, gate dielectric) but do not include metal connectors to the top gate, source and/or drain of the transistors. Devices of a standard CMOS stack may be integrated to various optical devices with electrical input via elements formed at the top layers during a BEOL process. There is currently no method for implementing photonic integration of PSRs at the bottom FEOL layers of a CMOS stack.